PREFACE. vii 



the broad facts of the case are of fundamental im- 

 portance; and, so far as these are concerned, I ven- 

 ture to hope that no error has slipped into my 

 statement of them. As for the details^ it must be 

 remembered, not only that some omission or mis- 

 take is almost unavoidable, but that new lights 

 come with new methods of investigation ; and that 

 better modes of statement follow upon the improve- 

 ment of our general views introduced by the gradual 

 widening of our knowledge. 



I sincerely hope that such amplifications and 

 rectifications may speedily abound; and that this 

 sketch may be the means of directing the attention of 

 observers in all parts of the world to the crayfishes. 

 Combined efforts will soon fm^nish the answers to 

 many questions which a single worker can merely 

 state; and, by completing the history of one group 

 of animals, secure the foundation of the whole 

 of biological science. 



In the Appendix, I have added a few notes re- 

 specting points of detail with which I thought it 



